The present invention relates to a cover film for dry film resists. More detail, the present invention relates to a cover film for dry film resists having excellent releasability and surface properties.
Recently, miniaturization of electronic devices has raised request for higher density of printed wiring boards, and it has become an urgent necessity to make finer the conductor patterns formed on the insulated substrate surface.
The dry film resist used in manufacture of the printed wiring boards has a three-layer structure of carrier film/photoresist/cover film, and polyethylene film has been used as the cover film for such dry film resists. However, thickness distribution and tendency to form fish-eyes of the polyethylene film obstruct the realization of higher density of the printed wiring boards.
Dry film resist is produced by coating a photoresist layer on a carrier film and, after drying the formed photoresist layer, laminating a cover film thereon. The photoresist is flexible when the cover film is laminated. Therefore, if unevenness is formed on the cover film surface due to fish-eyes, it is transferred to the photoresist layer to form similar unevenness on its surface, causing improper adhesion of the photoresist layer to the insulated substrate, which leads to a defective conductor pattern.
The polyethylene film is molded by melt extrusion, but because of high melt viscosity, it is hardly possible to filter the melt by a high-performance filter when the melt is extruded, so that the presence of fish-eyes or the like in the molded sheet is unavoidable. Further, the inflation method generally used for forming the polyethylene films is incapable of providing a uniform film thickness and also subject to the problem of deflection of film thickness.
Thus, it has increasingly become difficult to comply with the request for higher density of the printed wiring boards by using polyethylene for the cover film.
Japanese Patent Application Laid-Open (KOKAI) No. 6-297565 proposes use of a polyester film for dry film resists in place of a polyethylene film.
In the polyester film according to JP-A-6-297565, however, in order to provide the same flexibility as the polyethylene film for securing releasability, a release agent is coated on the film to afford the desired releasability and a copolymer component such as a long-chain aliphatic dicarboxylic acid and/or a polyolefin are blended to provide flexibility. Since the said film is unsatisfactory in releasability provided by the coating, it needs to impart flexibility to the base film. However, since a specific material is used for the base film, the above polyester film involved such problems as difficulties in regenerating the film and high production cost.
As a result of the present inventors"" earnest studies to solve the above problem, it has been found that a film comprising a base film and a wax-containing coating layer and, having a specific water droplet contact angle is suited as a cover film for dry film resists.
The present invention has been attained on the basis of the above finding.
The object of the present invention is to provide a cover film for dry film resists having excellent releasability and surface properties.
To attain the above aim, in an aspect of the present invention, there is provided a cover film for dry film resist comprising a base film and at least one wax-containing coating layer on at least one side of a base film, the water droplet contact angle of the surface of said coating layer being not less than 80xc2x0.
The present invention will be described in detail below.
In the cover film for dry film resists according to the present invention, the resin constituting the base film is preferably a polyester. The term xe2x80x9cpolyesterxe2x80x9d used in the present invention refers to highly crystalline linear saturated polyesters synthesized from aromatic dibasic acids or ester-forming derivatives thereof and glycols.
The dicarboxylic acids usable as a component of the polyesters in the present invention include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid, naphthalenedicarboxylic acid, paraphenylenedicarboxylic acid and derivatives thereof, alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and its derivatives, aliphatic dicarboxylic acids such as succinic acid, adipic acid, suberic acid, sebacic acid, dodecanedionic acid and derivatives thereof, and polyfunctional acids such as trimellitic acid, pyromellitic acid and derivatives thereof. The diols usable as another component of the polyesters include ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol, polyethylene glycol, polytetramethylene glycol, 1,4-cyclohexanedimethanol and derivatives thereof.
The preferred polyester for use in the present invention is polyethylene terephthalate.
The haze of the cover film for dry film resists according to the present invention is not more than 7%, preferably not more than 4%. If the haze exceeds 7%, it tends to become difficult to detect defects such as a coating hole in the coating of the photoresist.
The thickness of the cover film for dry film resists according to the present invention is preferably not more than 25 xcexcm, more preferably not more than 20 xcexcm, even more preferably not more than 16 xcexcm. If the cover film thickness is more than 25 xcexcm, the photoresist surface may be damaged when the cover film is released, because the film is too firm. The lower limit of the cover film thickness is usually 3.5 xcexcm.
In order to provide desired roughness to the surface of the obtained film in the present invention, it is preferable to add fine particles to the film or the coating layer. The particles of various materials such as silica, alumina, kaolin, calcium carbonate, barium salts, etc., can be used for this purpose.
The surface roughness of the coating layer is preferably 0.005 to 0.2 xcexcm, more preferably 0.02 to 0.1 xcexcm. If the surface roughness is less than 0.005 xcexcm, the winding properties of the obtained film tend to deteriorate, and if the surface roughness exceeds 0.2 xcexcm, air traps tend to form between the cover film and the photoresist to cause hardening of the resist by the reaction between oxygen and resist.
In the cover film for dry film resists according to the present invention, a wax is blended in the coating solution used for forming the coating layer. As such a wax, there can be used natural waxes such as vegetable waxes, animal waxes, mineral waxes and petroleum waxes, as well as synthetic waxes such as synthetic hydrocarbons, modified waxes and hydrogenated waxes. Of these waxes, polyolefin waxes are preferably used. For instance, the compounds having as the basic skeleton a polyolefin compound comprising a polymer or copolymer of an unsaturated hydrocarbon such as ethylene, propylene, 1-butene, 4-methyl-1-pentene or the like are used as a solution or a dispersion. More specific examples of such compounds include polyethylene, polypropylene, poly-1-butene, poly-4-methyl-1-pentene, ethylene-propylene copolymer, ethylene-1-butene copolymer, propylene-1-butene copolymer and such.
In the present invention, it is preferable to use polyolefins with an acid value of 10 to 50 having an active hydrogen group at the terminal, or to use polyethylene oxide or polypropylene oxide. The xe2x80x9cacid valuexe2x80x9d referred to herein means the number of mg of potassium hydroxide needed for neutralizing the free fatty acids contained in 1 g of the specimen. It is a value proportional to the amount of the free fatty acids contained in the wax. When using a polyolefin compound with an acid value of less than 10, although releasability of the cover film from the photoresist is satisfactory, the coating film strength of the coating layer tends to lower and part of the coating layer may be transferred to the resist layer after release of the cover film, causing a reduction of adhesion when the resist layer is laminated on the insulated substrate. In case of using a polyolefin compound with an acid value exceeding 50, although no transfer of the coating layer to the resist layer occurs, releasability from the photoresist may be deteriorated.
Specifically, a compound having as its basic skeleton a polyolefin compound comprising, for example, an ethylene or propylene polymer and/or copolymer and having an active hydrogen group such as carboxyl group, hydroxyl group or amino group at the terminal is used in the form of a solution or dispersion. Especially, polyethylene oxide wax, polypropylene oxide wax, ethylene oxide-propylene copolymer wax and the like are preferably used.
The wax (a polyolefin compound such as mentioned above) is blended in an amount of usually 1 to 100 parts by weight, preferably 20 to 100 parts by weight, based on 100 parts by weight of the coating solution used for forming the coating layer. If the amount of the wax (a polyolef in compound) blended is less than 20 parts by weight, releasability of the cover film from the photoresist tends to lower.
In order to coat the wax on the film with good appearance and improve adhesion between the coating layer and the film, it is preferable to mix a crosslinking agent in the coating solution. As such a crosslinking agent, it is possible to use, for example, melamine-type, amide-type and acrylamide-type compounds, epoxy compounds, polyisocyanates, blocked polyisocyanates, and carbodimide compounds.
The crosslinking agent is blended in an amount of usually 1 to 50 parts by weight, preferably 5 to 30 parts by weight, based on 100 parts by weight of the coating solution. When the amount of the crosslinking agent blended exceeds 30 parts by weight, releasability of the cover film from the photoresist tends to deteriorate, and when the amount of the crosslinking agent is less than 1 part by weight, coating film strength of the coating layer tends to lower.
In the said coating solution, in order to improve its coating performance, there may be contained lubricant, antistatic agent, defoaming agent, particles, etc., within limits not affecting the effect of the present invention.
As the coating means, it is possible to use various types of coating machines including reverse roll coater, gravure coater, rod coater, air doctor coater, etc., such as illustrated in Y. Harasaki: Coating System, Maki Shoten, 1979, and other types of coating equipment.
In the present invention, the coating layer can be provided by an in-line coating system in which coating is conducted in the film forming process, an off-line coating system in which coating is performed after formation of the film, or other systems, but the in-line coating system is preferred.
In-line coating is a method in which coating is conducted in the polyester film forming process, more specifically a method in which coating is performed at any suitable stage in the process from melt extrusion of polyester to take-up of the film after biaxial stretching and heat setting. Usually, coating is made on a substantially amorphous-state non-stretched sheet obtained by rapidly cooling the molten extrudate, or a monoaxially stretched film obtained by stretching the said sheet monoaxially in the longitudinal (machine) direction, or a biaxially stretched film before heat setting. Especially preferably, coating is performed on a monoaxially stretched film, and then the film is further stretched in the transverse direction. This method is advantageous in terms of production cost since film forming and drying of the coating layer can be accomplished simultaneously. Also, thin film coating is easy to perform because stretching is conducted after coating, and further, the coating layer and the polyester film adhere securely to each other since the heat treatment applied after coating is conducted at a high temperature which is inapplicable with the other methods.
The water droplet contact angle of the coating layer surface of the cover film according to the present invention is not less than 80xc2x0, preferably not less than 90xc2x0. If the contact angle is less than 80xc2x0, releasability of the cover film deteriorates, making the photoresist surface vulnerable to flawing when the cover film is released.